The present invention relates to an iridium oxide film, and more particularly, to an iridium oxide film used as an electrode of a stacked ferroelectric memory cell, and a method for fabricating such an iridium oxide film.
In recent years, research and development on ferroelectric films having the spontaneous polarization property have been actively done for success in commercialization of nonvolatile RAMs permitting high-speed write/read operation at a low operating voltage.
In semiconductor memories using a high dielectric film or ferroelectric film described above as a capacitor insulating film, if megabit-scale highly-integrated memories are desired, stacked memory cells are used in place of conventional planer memory cells.
The stacked memory cells have the following problem. A contact plug extends through an interlayer insulating film covering a field effect transistor. The face of the contact plug in contact with a lower electrode of a capacitor formed on the interlayer insulating film must be prevented from being oxidized during high-temperature heat treatment in an oxygen atmosphere, which is required for crystallization of an insulating metal oxide constituting the ferroelectric or high dielectric film.
To overcome the above problem, an iridium oxide film, a conductive oxide film, is normally formed between the contact plug and the lower electrode as an oxygen barrier film.
As a method for forming the iridium oxide film, reactive sputtering is proposed, in which an iridium metal target is subjected to sputtering and sputtered iridium is oxidized near a substrate (Journal of the Society of Photographic Science and Technology of Japan (1988), Vol. 51, No. 1, p. 3).
However, in the above publication, no examination is made on specific conditions for improving the oxygen barrier property of the iridium oxide film formed by reactive sputtering.
The conventional technique therefore fails to ensure improvement of the oxygen barrier property of the iridium oxide film.
An object of the present invention is providing a semiconductor memory device including an iridium oxide film as an oxygen barrier film for an electrode of a capacitor, in which improvement of the barrier property of the iridium oxide film is ensured.
To attain the above object, the first semiconductor memory device of the present invention includes: an interlayer insulating film formed on a semiconductor substrate; a contact plug formed to extend through the interlayer insulating film; and a capacitor formed on the interlayer insulating film, an electrode of the capacitor being connected with the contact plug, wherein the electrode has an iridium oxide film as an oxygen barrier film, and the average grain size of granular crystals constituting the iridium oxide film is a half or less of the thickness of the iridium oxide film.
According to the first semiconductor memory device of the invention, the average grain size of granular crystals constituting the iridium oxide film is a half or less of the thickness of the iridium oxide film. Therefore, diffusion paths for oxygen atoms in the iridium oxide film have curves. This makes it difficult for oxygen atoms to diffuse along grain boundaries, and thus the oxygen barrier property of the iridium oxide film improves with certainty.
In the first semiconductor memory device, the thickness of the iridium oxide film is preferably 200 nm or less.
By setting as described above, it is ensured that the average grain size of granular crystals constituting the iridium oxide film is a half or less of the thickness of the iridium oxide film.
The second semiconductor memory device of the invention includes: an interlayer insulating film formed on a semiconductor substrate; a contact plug formed to extend through the interlayer insulating film; and a capacitor formed on the interlayer insulating film, an electrode of the capacitor being connected with the contact plug, wherein the electrode has an iridium oxide film as an oxygen barrier film, and the iridium oxide film has a plurality of layers different in average crystal grain size from each other.
According to the second semiconductor memory device of the invention, the iridium oxide film as an oxygen barrier film has a plurality of layers different in average crystal grain size from each other. Therefore, it is difficult for oxygen atoms to diffuse along grain boundaries of crystals constituting one of the layers having a smaller average crystal grain size, and thus the oxygen barrier property of the iridium oxide film improves. In addition, the layer having a larger average crystal grain size serves to improve the adhesion to a film in contact with the oxygen barrier film.
In the second semiconductor memory device, preferably, a lower layer of the plurality of layers has a comparatively large average crystal grain size, and an upper layer of the plurality of layers has a comparatively small average crystal grain size.
With the above construction, the upper layer can block diffusion of oxygen atoms coming from the upper side of the oxygen barrier film, while the lower layer can improve the adhesion to a film formed on the lower side of the oxygen barrier film.
Preferably, the thickness of the lower layer is 30 nm or less, and the thickness of the upper layer is 200 nm or less.
By setting as described above, it is ensured that the upper layer blocks diffusion of oxygen atoms while the lower layer improves the adhesion to a film formed on the lower side of the oxygen barrier film.
The third semiconductor memory device of the present invention includes: a field effect transistor formed on a semiconductor substrate; an interlayer insulating film formed to cover the field effect transistor; a contact plug formed to extend through the interlayer insulating film and connected with a source region or a drain region of the field effect transistor; and a capacitor formed on the interlayer insulating film, a lower electrode of the capacitor being connected with the contact plug, wherein the lower electrode has an iridium oxide film as an oxygen barrier film, and the average grain size of granular crystals constituting the iridium oxide film is a half or less of the thickness of the iridium oxide film.
According to the third semiconductor memory device of the invention, the average grain size of granular crystals constituting the iridium oxide film is a half or less of the thickness of the iridium oxide film. Therefore, diffusion paths for oxygen atoms in the iridium oxide film have curves. This makes it difficult for oxygen atoms to diffuse along grain boundaries, and thus the oxygen barrier property of the iridium oxide film improves with certainty.
In the third semiconductor memory device, the thickness of the iridium oxide film is preferably 200 nm or less.
By setting as described above, it is ensured that the average grain size of granular crystals constituting the iridium oxide film is a half or less of the thickness of the iridium oxide film.
The fourth semiconductor memory device of the present invention includes: a field effect transistor formed on a semiconductor substrate; an interlayer insulating film formed to cover the field effect transistor; a contact plug formed to extend through the interlayer insulating film and connected with a source region or a drain region of the field effect transistor; and a capacitor formed on the interlayer insulating film, a lower electrode of the capacitor being connected with the contact plug, wherein the lower electrode has an iridium oxide film as an oxygen barrier film, and the iridium oxide film has a plurality of layers different in average crystal grain size from each other.
According to the fourth semiconductor memory device of the invention, the iridium oxide film as an oxygen barrier film has a plurality of layers different in average crystal grain size from each other. Therefore, it is difficult for oxygen atoms to diffuse along grain boundaries of crystals constituting one of the layers having a smaller average crystal grain size, and thus the oxygen barrier property of the iridium oxide film improves. In addition, the layer having a larger average crystal grain size serves to improve the adhesion to a film in contact with the oxygen barrier film.
In the fourth semiconductor memory device, preferably, a lower layer of the plurality of layers has a comparatively large average crystal grain size, and an upper layer of the plurality of layers has a comparatively small average crystal grain size.
With the above construction, the upper layer can block diffusion of oxygen atoms coming from the upper side of the oxygen barrier film, while the lower layer can improve the adhesion to a film formed on the lower side of the oxygen barrier film.
Preferably, the thickness of the lower layer is 30 nm or less, and the thickness of the upper layer is 200 nm or less.
By setting as described above, it is ensured that the upper layer blocks diffusion of oxygen atoms while the lower layer improves the adhesion to a film formed on the lower side of the oxygen barrier film.
The first fabrication method of the present invention is a method for fabricating a semiconductor memory device including: an interlayer insulating film formed on a semiconductor substrate; a contact plug formed to extend through the interlayer insulating film; and a capacitor formed on the interlayer insulating film, an electrode of the capacitor being connected with the contact plug, the electrode having an iridium oxide film as an oxygen barrier film. The method includes the step of: forming the iridium oxide film by reactive sputtering with oxygen gas and argon gas fed into a reaction chamber, a target including indium being placed in the chamber, wherein the reactive sputtering is performed under the condition satisfying an expression x/(x+y) greater than 0.5 wherein x is the actual partial pressure of the oxygen gas in the reaction chamber and y is the actual partial pressure of the argon gas in the reaction chamber.
The second fabrication method of the present invention is a method for fabricating a semiconductor memory device including: a field effect transistor formed on a semiconductor substrate; an interlayer insulating film formed to cover the field effect transistor; a contact plug formed to extend through the interlayer insulating film and connected with a source region or a drain region of the field effect transistor; and a capacitor formed on the interlayer insulating film, a lower electrode of the capacitor being connected with the contact plug, the lower electrode having an iridium oxide film as an oxygen barrier film. The method includes the step of: forming the iridium oxide film by reactive sputtering with oxygen gas and argon gas fed into a reaction chamber, a target including indium being placed in the chamber, wherein the reactive sputtering is performed under the condition satisfying an expression x/(x+y) greater than 0.5 wherein x is the actual partial pressure of the oxygen gas in the reaction chamber and y is the actual partial pressure of the argon gas in the reaction chamber.
According to the first and second methods for fabricating a semiconductor memory device of the invention, the ratio of oxygen gas to argon gas fed into the reaction chamber is made large. This changes the atoms colliding against the target for causing sputtering from argon atoms to oxygen atoms, and thus decreases the kinetic energy of sputtered grains. With decreased kinetic energy, the average crystal grain size of grains constituting the iridium oxide film is made small.
Therefore, according to the first and second fabrication methods, diffusion paths for oxygen atoms in the iridium oxide film have curves. This makes it difficult for oxygen atoms to diffuse along grain boundaries, and thus the oxygen barrier property of the iridium oxide film improves with certainty.
In addition, since the iridium oxide film can be formed on the substrate having a temperature kept equal to or higher than room temperature, the adhesion between the contact plug and the lower electrode of the capacitor improves.
The third fabrication method of the present invention is a method for fabricating a semiconductor memory device including: an interlayer insulating film formed on a semiconductor substrate; a contact plug formed to extend through the interlayer insulating film; and a capacitor formed on the interlayer insulating film, an electrode of the capacitor being connected with the contact plug, the electrode having an iridium oxide film as an oxygen barrier film, the iridium oxide film having a plurality of layers different in average crystal grain size from each other. The method includes the step of: forming the iridium oxide film by reactive sputtering with oxygen gas and argon gas fed into a reaction chamber, a target including indium being placed in the chamber, wherein the reactive sputtering is performed under the condition satisfying an expression x/(x+y) less than 0.5, wherein x is the actual partial pressure of the oxygen gas in the reaction chamber and y is the actual partial pressure of the argon gas in the reaction chamber, in the first stage, and under the condition satisfying an expression x/(x+y) greater than 0.5 in the subsequent second stage.
The fourth fabrication method of the present invention is a method fabricating a semiconductor memory device comprising: a field effect transistor formed on a semiconductor substrate; an interlayer insulating film formed to cover the field effect transistor; a contact plug formed to extend through the interlayer insulating film and connected with a source region or a drain region of the field effect transistor; and a capacitor formed on the interlayer insulating film, a lower electrode of the capacitor being connected with the contact plug, the lower electrode having an iridium oxide film as an oxygen barrier film, the iridium oxide film having a plurality of layers different in average crystal grain size from each other. The method includes the step of: forming the iridium oxide film by reactive sputtering with oxygen gas and argon gas fed into a reaction chamber, a target including indium being placed in the chamber, wherein the reactive sputtering is performed under the condition satisfying an expression x/(x+y) less than 0.5 wherein x is the actual partial pressure of the oxygen gas in the reaction chamber and y is the actual partial pressure of the argon gas in the reaction chamber, in the first stage, and under the condition satisfying an expression x/(x+y) greater than 0.5 in the subsequent second stage.
According to the third and fourth methods for fabricating a semiconductor memory device of the invention, in the first stage, the reactive sputtering is performed under the condition satisfying an expression x/(x+y) less than 0.5. Therefore, the kinetic energy of sputtered grains is large, and this improves the adhesion between the contact plug and the lower electrode. In the second stage, the reactive sputtering is performed under the condition satisfying an expression x/(x+y) greater than 0.5. Therefore, the kinetic energy of sputtered grains is small, and this decreases the average crystal grain size of grains constituting the iridium oxide film.
Thus, according to the third and fourth fabrication methods, the adhesion between the contact plug and the electrode of the capacitor improves, and also the oxygen barrier property of the iridium oxide film improves.
Preferably, the first to fourth methods for fabricating a semiconductor memory device further include the step of subjecting the iridium oxide film to heat treatment at a temperature in a range of 500xc2x0 C. to 600xc2x0 C. in a nitrogen atmosphere.